There is a need for such large conductors for commercial, military, and aerospace applications, such as test equipment and submarine wiring, airframe routing of communication and control signals, control "black" box interconnectors, and television and radio equipment signal routing. A desirable product would have light weight, small size, and excellent electrical performance. It has been difficult in the past, however, to achieve this combination of desirable properties owing to problems associated with extruding thick layers of porous insulation over large electrical conductors consistently without loss of electrical performance characteristics.
Early methods comprised spacing the conductor from the surrounding metal screen by braiding flexible cords, tubes or strands of insulation in a pattern between the two metal layers and optionally filling the space between the strands with an insulating gas or insulating liquid, such as described in U.S. Pat. Nos. 2,488,211 to Lemon and 2,585,484 to Menes. Another method utilized was to surround the center conductor of a cable with insulating tubes, which could be of various shapes, and bind them by a winding of insulating tape to the conductor, then apply a metallic shield, much as shown in U.S. Pat. No. 3,126,436.
A method differing in kind was a process to extrude a layer of PTFE insulation onto a conductor, stretch, and sinter in a single pass to yield an electric conductor covered by a low density PTFE insulation. This process, shown in U.S. Pat. No. 4,529,564, involved a complex way to move the conductor and insulation at differing rates to stretch the insulation, and to heat the stretched insulation to heat-set its structure at about the time the rate of insulation movement caught up to that of the conductor.